The factor VIII in factor VIII concentrates is being characterized by activity and antigen assays and by immunoblotting. The availability of factor VIII concentrates of increasing purity, including recombinant factor VIII, has raised concern about potency testing and potential immunogenicity resulting in increased numbers of hemophiliacs who form antibodies to factor VIII used as therapy. The potency of these products can be influenced by the state of activation of the factor VIII. This activated state can possibly be detected by the presence of factor VIII fragments known to be formed by thrombin cleavage, by an increase in activity relative to antigen, or by assays relatively specific for activated factor VIII. All factor VIII concentrates available in the U.S. or under review by FDA for licensure are being studied in an attempt to characterize their factor VIII and determine if there are differences among then in activation.